There are many instances in which it is necessary to draw a circle or an arc of a circle with a radius larger than that normally allowed by a standard compass or divider. In such cases, it has been necessary to either provide a special tool so that the desired radius can be obtained, or else to modify some existing tool for that purpose. For example, several years ago it was possible to drill small holes at predetermined locations in a standard carpenter's folding rule. A nail could then be passed through one of the holes to act as a center point. A pencil or other scribing device passed through a second hole could then be moved to form the desired arc.
Over the course of years, however, the folding rule has generally been replaced by the more convenient and smaller measuring tape. Such tapes normally utilize a housing enclosed on all sides and having a slit in one wall through which a tape may be withdrawn temporarily to take a measurement. The end of the tape is usually provided with a holder to facilitate taking an outside measurement by one person alone.
In recent years, the tapes used with such measuring devices have been provided with a uniform convex configuration throughout the length thereof. This configuration imparts characteristics which facilitate the coiling of the tape within the housing, as well as strengthening of the tape when it is pulled from the housing.
As such measuring tapes became more readily available, several proposals have been made for adaptation of those tapes for use as compasses. Examples of such proposals might be found, by way of example, in U.S. Pat. Nos. 2,804,688; 3,120,059; 3,526,964; and 3,577,641.
Each of the devices depicted in those four patents has disclosed a measuring tape in which the housing is constructed during its initial manufacture in such a manner as to allow a user to draw or scribe a circle about a fixed point. In each case, the fixed point is mounted on or within the housing of the measuring tape by means of structure developed during the initial production of the device.
On the other hand, there have been some attempts to develop structure which could be added to a standard measuring tape in order to convert such a tape to a device which would scribe arcs of circles. Such devices have been shown in U.S. Pat. Nos. 2,651,843 and 3,063,157. However, the former patent requires use of two clips which may be temporarily positioned at spaced locations on a measuring tape. Thus, when it is desired to retract the tape, at least one of the clips must be removed and stored elsewhere.
The latter patent, on the other hand, discloses structure which may be temporarily mounted on the bottom of a measuring tape case or housing. The structure includes a selectively movable arm which forms a pivot point when the arm is swung to an operative position. However, the structure into which the casing is placed thus alters the effective length of the bottom of the housing. Stated another way, the housing dimension between the tape withdrawal slot and the opposite housing wall is thus increased.
In many cases, the distance between the measuring tape slot in one wall of the housing and the opposite wall of the housing is precisely determined during manufacture so that the housing may be used in coordination with the tape in order to facilitate the measurement of an inside dimension. Consequently, when any structure alters the position of the back end of the case, relative to the tape slot, that alteration reduces the utility of the case and that utility can only be restored by removal of the altering structure.